Charge pump circuits are frequently used in a semiconductor integrated circuit to provide an internal voltage that is higher than an external voltage provided to the semiconductor integrated circuit from an external power supply, such as a battery. Charge pump circuits are also used to provide a voltage of reverse polarity. Charge pump circuits are particularly useful in FLASH and EEPROM non-volatile memories, but are gaining more and more acceptance in analog circuits in order to increase dynamic range and to simplify circuit design.
FIG. 1 illustrates a conventional Dickson charge pump 10 which uses multiple stages of switched-capacitor circuits. Each of the stages includes a capacitor and a diode-connected NMOS transistor. A first Dickson charge pump stage includes an NMOS transistor ND1 and a capacitor C1. A second Dickson charge pump stage includes an NMOS transistor ND2 and a capacitor C2. A third Dickson charge pump stage includes an NMOS transistor ND3 and a capacitor C3. A fourth Dickson charge pump stage includes an NMOS transistor ND4 and a capacitor C4. A fifth Dickson charge pump stage includes an NMOS transistor ND5 and a capacitor C5. The NMOS transistors ND1, ND2, ND3, ND4, ND5 each have their bulk or substrate connected to ground, their drain and gate connected together to a stage capacitor, and their source connected to the capacitor of the next stage. Two oppositely phased clock signals, ΦA and ΦB are used for pumping charge from stage to stage. The maximum gain per stage of the Dickson charge pump 10 is (VDD-VT), where VT is the threshold voltage of an NMOS device. For n stages, the gain of a conventional Dickson charge pump is n(VDD-VT). The gain is thus reduced by n times the VT threshold voltage.
For some applications, the conventional Dickson charge pump 10 has a number of drawbacks. For instance, the number of stages that can be cascaded is limited by the amount of the voltage drop increase between the source and the bulk of an NMOS device resulting in a dramatic VT increase on the last stages. Another significant drawback is that high voltage transistors with thick oxide layers are necessary to sustain a large voltage drop between gate and bulk in a reliable way. This makes it difficult to design conventional Dickson charge pumps using thin oxide, low voltage standard devices which can sustain a maximum drop of only VDD.